The present invention relates generally to optical fibers, and in particular to a device which provides a monitoring mechanism by using partial transmissions from the taps in a tapped delay line filter to provide enhancement of the filter""s performance in real-time operation.
Actively configurable optical filters have many potential applications in optical communications, such as temporal pulse compression, dispersion slope compensation and spectral shaping. Spectral shaping is of particular interest in connection with optical amplifiers. The gain profile of a typical optical amplifier may be affected by numerous factors such as, for example, fiber length, fiber composition, splice losses, as well as the population inversion level of the dopant atoms (e.g. erbium) providing the gain. Having a relatively large optical signal at the input of the amplifier tends to deplete the excited electron population, resulting in a different spectral gain profile than that for a small input signal. Average gain also varies with input signal power.
One approach to dealing with this effect is to operate the amplifier at a constant population inversion, with a fixed spectral filter that provides gain flattening over the operating wavelength range. In this approach, the inversion level is held fixed by selectively attenuating the input signal until the desired inversion level is achieved. However, there may be circumstances where operating the amplifier at a constant gain level, rather than a constant inversion level, would be advantageous. By utilizing an actively configurable spectral filter, an amplifier could be used in a constant-gain mode, rather than constant inversion level, by flattening the gain profile of the amplifier for arbitrary inversion levels. One approach for realizing such a configurable spectral filter is to spatially disperse the various wavelength components of the input signal via a diffraction grating, then spatially modulate the dispersed components via a spatial light modulator or acoustically-formed phase grating. The polarization dependence and diffractive losses of the grating could be a drawback in some applications, however. Prisms could be used instead of a diffraction grating to eliminate polarization dependence and to decrease angular dispersion. However, prisms require an increase in package size, which makes their use undesirable in many applications.
In xe2x80x9cPlanar Tapped Delay Line Based, Actively Configurable Gain-Flattening Filterxe2x80x9d, Proceedings of the 26th European Conference on Optical Communication, Vol. 3, pages 21-22 (2000), A. Ranalli and B. Fondeur have shown that the filters of the title, when configured using thermo-optic perturbation of the index of refraction in silica channel waveguides, are suitable for applications requiring spectrally slow optical transfer functions. A finite impulse response (xe2x80x9cFIRxe2x80x9d) version of the filter achieves a desired optical transfer function (xe2x80x9cOTFxe2x80x9d) by splitting the input signal into several taps, delaying each by a sampling interval with respect to its neighbor tap, imparting a nearly wavelength-independent phase shift to each tap, and then recombining the delayed components in the same ratios into which they were split. As a result of such procedures, a given filter configuration can be characterized by specifying each tap""s power split ratio and delay perturbation, or equivalently, its phase.
In an ideal situation, the desired filter state can be achieved in an xe2x80x9copen-loopxe2x80x9d manner; for example, by calibrating splitter structures and phase-shifters, and then relying on the calibration data to reproduce any arbitrary state. However, if one wishes to monitor the output spectrum, or if the calibration data is not sufficiently reliable to perform as a valid indicator of the state of the device, then external monitoring must be provided.
One aspect of the invention is an apparatus for changing the spectral profile of an optical signal that includes a plurality of optical paths, each being associated with a delay element which imparts a predetermined delay to a signal propagating through the optical path. A plurality of couplers is configured to split the optical signal among the plurality of optical paths according to a predetermined splitting ratio. The predetermined delays and predetermined splitting ratios are chosen so as to effect the desired change to the spectral profile.
In another aspect, the delay elements may include portions of the waveguide that have an altered index of refraction. The index of refraction may, in yet another aspect, be reversibly alterable.
In another aspect, the invention includes a method for modifying the spectral distribution of an optical signal which includes the steps of splitting the signal among a plurality of optical paths, each having an output end, so that a portion of the signal propagates on each of the optical paths; changing the relative phases of the portions of the signal propagating through the optical paths; modifying the amplitudes of the portions of the signal propagating through the optical paths; and combining the portions of the signal at the output ends of the optical paths. The relative phase changes and the amplitude modifications have magnitudes that result in a desired spectral distribution when the portions of the signal at the output ends of the optical paths are combined.
In another aspect, the invention includes a method for modifying the spectral distribution of an optical signal which includes the steps of splitting the signal among a plurality of optical paths according to predetermined splitting ratios, and changing the relative phases of the portions of the signal propagating through different ones of the optical paths; the relative phase changes and the splitting ratios having values which result in a desired spectral distribution when the portions of the signal at the output ends of the optical paths are combined; and combining the portions of the signal at the output ends of the optical paths.
In another aspect, the invention includes an optical communication system comprising an optical amplifier and a spectral filter that includes an input waveguide and a plurality of tapped delay lines. Each of the tapped delay lines has a delay element configurable to impart a predetermined delay to the signal propagating down the delay line. Couplers split an optical signal, propagating on the input waveguide, among the tapped delay lines. The delays imparted to the signals on each line are chosen so as to effect the desired change to the spectral profile.
In another aspect, the invention is directed to providing a monitoring mechanism by using partial transmissions from the taps themselves as a tapped delay line, diffractive array. The time-averaged diffractive pattern from the taps can then reveal information that can then be used to enhance the estimate of the filter""s instantaneous state.
The claimed embodiments of the invention provide spectral shaping which is polarization-independent, and which, in certain embodiments, may be actively configurable or permanently configured. The spectral filtering of the invention may, if desired, be provided in a planar device, which is compact, reliable and economically fabricated. It is particularly well suited to use as a gain-flattening filter for an optical amplifier, which allows the amplifier to be used in a constant gain mode. Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention. Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.